Promoters are vital molecular tools that have been applied widely in plant biotechnology to control the expression of introduced genes. There are many applications for promoters in driving gene expression in plant tissues. These include the synthesis of scoreable and selectable markers to identify transgenic plants (Jefferson et al., 1987 [1]; Wohlleben et al., 1988 [2]) and the over-expression of control point enzymes to modify metabolic flux through key pathways, so affecting the yields of important plant products (Nessler, 1994 [3]; Lessard et al., 2002 [4]). Other uses of plant promoters include the expression of genes conferring resistance to pests, thus conferring protection (Estruch et al., 1997 [5]), and the expression of non-native enzymes to facilitate the production of foreign metabolites in particular plant species (Poirier et al., 1995 [6]; Ye et al., 2000 [7]). A further application of plant promoters is to over-express controlling regulatory genes affecting aspects of plant physiology such as flowering time and so modify plant growth characteristics (Weigel and Nilsson, 1995 [8]). Promoters are also used to repress the expression of specific genes by driving the synthesis of interfering RNA species (Waterhouse et al., 2001 [9]), thus affecting plant metabolic and developmental pathways (Yu and Kumar, 2003 [10]). Although high levels of expression may not be necessary for all of the above applications, there is clearly a need for promoters showing activity in plant tissues.
Apart from these and other applications of promoters to modify plant traits, promoters are also required for plants to act as production systems for heterologous proteins. Plants have been used to produce a wide range of recombinant proteins of potential economic and/or medicinal importance. These include research chemicals (Hood et al., 1997 [11]; Zhong et al., 1999 [12]), processing enzymes that are used, for example, in the pharmaceutical industry (Woodard et al., 2003 [13]), industrial enzymes that are deployed in large-scale processing operations such as bleaching (Hood et al., 2003 [14]; Bailey et al., 2004 [15]), candidate vaccine antigens for animal or plant disease prevention (Mason et al., 1992 [16]; Haq et al., 1995 [17]; Carrillo et al., 1998 [18]; Streatfield et al., 2001 [19]), and therapeutic pharmaceuticals including antibodies (Daniell et al., 2001 [20]; Hood et al., 2002 [21]). The expressed proteins may either be purified from the plant tissues (Hood et al., 1997 [11]; Woodard et al., 2003 [13]) or, if as with vaccines the final application allows it, the recombinant plant material may be processed into a suitable form for use or even deployed directly (Streatfield et al., 2002 [22]; Lamphear et al., 2002 [23]). For these and other protein products to be produced in plant systems it is necessary that promoters drive a sufficiently high level of expression to ensure commercial viability.
Spatial and temporal control is also often important in driving gene expression in plants. For example selectable and scoreable markers must be expressed at a suitable time and in an appropriate tissue to allow for screening, and controlling enzymes and regulatory factors must be produced in metabolically active and physiologically responsive tissues, respectively. Similarly, genes conferring host protection must be expressed in the target tissues for the pathogen or pest, and plant produced protein products should be expressed in tissues suitable for protein accumulation and storage. Furthermore, since certain protein products may have detrimental affects on plant health and yield when expressed in metabolically active plant tissues that are essential for survival and growth, promoters may be favored that are active in the chosen plant storage tissues but show low or no activity in other, non-storage tissues.
Promoters that preferentially express relatively high levels of foreign proteins in tissues suitable for stable protein accumulation and storage are particularly useful for commercial protein production. The seed tissues of the cereals are especially well suited to the large-scale production of recombinant proteins. Thus, there is a requirement for promoters that show a seed tissue preferred expression pattern in plants and particularly cereals and drive relatively high levels of protein accumulation in these tissues.
Several promoters of plant and plant pathogen (bacterial and viral) origin have been used to direct transgene expression in plants. Prominent examples include the French bean beta-phaseolin promoter (Bustos et al., 1989 [24]), the mannopine synthase promoter of Agrobacterium tumefaciens (Leung et al., 1991 [25]), and the 35S promoter of cauliflower mosaic virus (Guilley et al., 1982 [26]). These and several other promoters in widespread use in plants were originally developed and utilized in dicot species. Promoter sequences from one species are predictably used in other species (see discussion below). The cereals comprise particularly important crops and there is therefore a pressing need for promoters that have high activity and/or tissue preference in monocots. Cereals, such as grasses, are cultivated for their grain. Since the nutritional value of cereals is in their seeds, and these tissues are also well suited for recombinant protein accumulation and storage, promoters that are active in cereal seed tissues are especially useful.
Two broad classes of promoters are typically deployed: constitutive and tissue preferred. Constitutive promoters, such as maize polyubiquitin-1 drive expression in the seed but also in other tissues (Christensen et al., 1992 [27]). A drawback with such constitutive promoters is that expression in tissues other than seed storage tissues may result in plant health being compromised, for example if a potentially toxic protein is expressed in metabolically active tissues required for germination or growth (Hood et al., 2003 [14]). Furthermore, constitutive expression may result in the expressed foreign protein being synthesized in pollen grains and thus being difficult to contain. By contrast, seed preferred promoters limit all or the bulk of transgene expression to seed tissues, so avoiding such concerns. Tissue preferred expression can include seed preferred expression. An example of one such promoter providing seed preferred expression is the phaseolin promoter. See, Bustos et al. (1989) [24].
The principle tissue types in maize seeds are the embryo, the endosperm including a surrounding aleurone cell layer and the maternally derived pericarp. Of these, the endosperm and to a lesser extent the embryo, comprise most of the volume of the seed. Thus, endosperm and embryo promoters are particularly important for modifying seed characteristics and contents. The proximal 1.1 kb of a maize 27 kD γ-zein promoter (Russell and Fromm, 1997 [28]) and the proximal 1.45 kb of a maize globulin-1 promoter (Belanger and Kriz, 1991 [29]; Genbank accession L22344) are prominent examples of seed preferred promoters that have been used to express transgenes in the seeds of monocots.
However, despite these examples, there is currently a very limited repertoire of promoters for preferentially expressing foreign or heterologous proteins or transcripts in the seed tissues of plants, and in particular, cereals. There is a need for further promoters that express transgenes at similar or higher levels to those currently deployed and with similar or improved tissue specificity. The best promoters would facilitate the expression of foreign proteins in seeds at higher levels than are currently achieved, while restricting expression specifically or predominantly to seed tissues. Also, a range of new promoters would allow the expression of multiple copies of a single transgene in seeds without the need to repeatedly use the same promoter. This should reduce silencing phenomena associated with promoter methylation (De Wilde et al., 2000 [30]), and thereby it should also serve to boost expression. Similarly, multiple distinct transgenes could be simultaneously expressed from different promoters in seed tissues, allowing more complex traits and foreign protein products to be reliably introduced into seeds.